A367431 - OEIS

%I #19 Nov 18 2023 18:13:47

%S 12,3,45,67,891011,1213,14151617,1819,20212223,242526272829,3031,

%T 323334353637,38394041,4243,44454647,484950515253,545556575859,6061,

%U 626364656667,68697071,7273,747576777879,80818283,848586878889,9091929394959697,9899100101,102103

%N Write down the positive integers. To obtain the terms of the sequence, concatenate groups of these so that the last number of each concatenated group is a prime.

%H Michael S. Branicky, <a href="/A367431/b367431.txt">Table of n, a(n) for n = 1..10000</a>

%o (Python)

%o from sympy import prime

%o from itertools import count, islice

%o def a(n):

%o     return int("".join(map(str, range(1 if n<2 else prime(n-1)+1, prime(n)+1))))

%o print([a(n) for n in range(1, 31)]) # _Michael S. Branicky_, Nov 18 2023

%o (Python) # faster for generating initial segment of sequence

%o from sympy import isprime

%o from itertools import count, islice

%o def agen():

%o     cat = ""

%o     for i in count(1):

%o         cat += str(i)

%o         if isprime(i): yield int(cat); cat = ""

%o print(list(islice(agen(), 30))) # _Michael S. Branicky_, Nov 18 2023

%Y Cf. A000027, A000040.

%K nonn,base

%O 1,1

%A _Tamas Sandor Nagy_, Nov 18 2023